The REgolith X-Ray Imaging Spectrometer (REXIS) for
OSIRIS-REx: identifying regional elemental enrichment on
asteroids
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Citation
Allen, Branden, Jonathan Grindlay, Jaesub Hong, Richard P.
Binzel, Rebecca Masterson, Niraj K. Inamdar, Mark Chodas, et
al. “The REgolith X-Ray Imaging Spectrometer (REXIS) for
OSIRIS-REx: Identifying Regional Elemental Enrichment on
Asteroids.” Edited by Mark A. Kahan and Marie B. Levine.
Optical Modeling and Performance Predictions VI (September
27, 2013). © 2013 SPIE
As Published
http://dx.doi.org/10.1117/12.2041715
Publisher
SPIE
Version
Final published version
Accessed
Wed May 25 22:08:53 EDT 2016
Citable Link
http://hdl.handle.net/1721.1/85182
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The REgolith X-Ray Imaging Spectrometer (REXIS) for
OSIRIS-REx: Identifying Regional Elemental Enrichment on
Asteroids
Branden Allena , Jonathan Grindlaya , Jaesub Honga , Richard P. Binzelb , Rebecca Mastersonc ,
Niraj K. Inamdarb,c , Mark Chodasc , Matthew W. Smithc , Marshall W. Bautzd ,
Steven E. Kisseld , Joel Villasenord , Miruna Oprescua , Nicholas Indunia ,
a Harvard
College Observatory, 60 Garden Street, Cambridge, MA 02138, USA
of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of
Technology, Cambridge, MA 02139, USA
c Space Systems Lab, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
d Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology,
Cambridge, MA 02139, USA
b Department
ABSTRACT
The OSIRIS-REx Mission was selected under the NASA New Frontiers program and is scheduled for launch in
September of 2016 for a rendezvous with, and collection of a sample from the surface of asteroid Bennu in 2019.
101955 Bennu (previously 1999 RQ36 ) is an Apollo (near-Earth) asteroid originally discovered by the LINEAR
project in 1999 which has since been classified as a potentially hazardous near-Earth object. The REgolith X-Ray
Imaging Spectrometer (REXIS) was proposed jointly by MIT and Harvard and was subsequently accepted as a
student led instrument for the determination of the elemental composition of the asteroid’s surface as well as the
surface distribution of select elements through solar induced X-ray fluorescence. REXIS consists of a detector
plane that contains 4 X-ray CCDs integrated into a wide field coded aperture telescope with a focal length of
20 cm for the detection of regions with enhanced abundance in key elements at 50 m scales. Elemental surface
distributions of approximately 50-200 m scales can be detected using the instrument as a simple collimator. An
overview of the observation strategy of the REXIS instrument and expected performance are presented here.
1. THE OSIRIS-REX MISSION
The Origins, Spectral Interpretation, Resource Identification, Security, Regolith, Explorer (OSIRIS-REx) [1, 2]
is a multifaceted asteroid regolith sample return mission chosen as part of NASA’s New Frontiers Program for
the characterization of a 101955 Bennu (previously designated 1999 RQ36 ). OSIRIS-REx is equipped with 5
separate instruments:
1. The OSIRIS-REx Camera Suite (OCAMS) which consists of three separate visible telescopes for the
mapping and sample site selection on the asteroid at visible wavelengths [3].
2. The OSIRIS-REx Laser Altimeter (OLA) which utilizes scanning LIDAR to provide high resolution topographical measurements of the asteroid surface [4].
3. Characterization of the asteroid spectra at visible and infrared wavelengths will be carried out using the
OSIRIS-REx Visible and IR Spectrometer (OVIRS) [5, 6].
4. Thermal emission spectral maps will be provided by the OSIRIS-REx Thermal Emission Spectrometer
(OTES).
5. The REgolith X-Ray Imaging Spectrometer (REXIS) will determine the global elemental abundances and
search for anisotropies in the composition of Bennu.
Further author information: (Send correspondence to Branden Allen; E-Mail: ballen@cfa.harvard.edu)
Optical Modeling and Performance Predictions VI, edited by Mark A. Kahan, Marie B. Levine, Proc. of SPIE Vol. 8840,
88400M · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2041715
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The spacecraft also carries the Touch-And-Go Sample Acquisition Mechanism (TAGSAM) for the retrieval of
no less than 60 g of regolith from the surface of Bennu.
The purpose and primary objectives of the OSIRIS-REx mission are the return of a regolith sample, the global
characterization of the asteroid’s morphology and composition, measurement of the properties of the sample site
at sub-centimeter scales, and threat assessment with regard to potential future impacts with the Earth. The
threat assessment will be carried out by preforming a field Yarkovsky test which will be used to refine future
orbital projections. Aside from its power to extinguish life Bennu is also potentially important for advancing
our understanding of planet formation, in particular our understanding of the origin of organic material which is
of fundamental importance for the development of life on Earth (see §2). The measurements made in orbit will
also be used to cross-calibrate observations carried out from the vicinity of the Earth which should contribute
to added precision in the measurement of asteroid properties in general.
OSIRIS-REx is scheduled for launch in September of 2016, with a backup date scheduled one year later. After
a gravity assist maneuver with the Earth in late 2017 OSRIS-REx will enter the asteroid approach phase in late
2019 and carry out a preliminary survey for small natural satellites which could pose a danger to the mission.
Once it is determined to be safe to proceed OSIRIS-REx will settle into an orbit around Bennu and primary
science operations will commence. Observations with the REXIS (see §3) will be performed in orbital phase B
at a nominal altitude of 720 m above the surface of the asteroid in order to assist with the selection of potential
sample retrieval sites by characterizing the global elemental abundance of Bennu and searching for regions of
anomalous composition. After selection of a sample site OSIRIS-REx will descend to the surface of Bennu and
collect up to 2.2 kg of regolith. OSIRIS-REx will depart the asteroid in March of 2021 with its cargo which will
land with the sample return capsule (SRC) at the Utah Test and Training Range (UTTR) in September of 2023.
2. TARGET: NEAR EARTH ASTEROID 101955 BENNU
101955 Bennu (1999 RQ36 ), was selected by the
OSIRIS-REx team for its accessibility, relatively high
impact probability, and it is a B-Type asteroid: a class
that likely samples the early chemistry of the solar system.
Bennu was originally detected by the Lincoln NearEarth Asteroid Program (LINEAR) in 1999 [7] and
subsequently was determined to have the highest impact hazard rating based on the Palermo Technical
Scale (c.f. [8]) with a cumulative value of −1.12 and a
maximum value of −1.52; for comparison the median
values drawn from a sample of 444 asteroids currently
tracked by the JPL Sentry System are Pcumulative =
−6.225 ± 1.490 and Pmaximum = −6.545 ± 1.474 where
the errors stated here are the RMS values calculated
over the sample [9]. The cumulative impact probability with the Earth is 7.1 × 10−4 with 8 potential
impacts between 2169 and 2199. The orbit of Bennu
is in a psudo-resonance with the Earth’s orbit and consequently makes close passes at intervals of approxi- Figure 1: A comparison of 101955 Bennu with the Saturn
mately 6 years; since its discovery ground based radar 5 rocket. Bennu has a mean radius of 246 ± 10 m, the
observations have been performed during the close en- Saturn 5 had a height of 110 m.
counters of 1999, 2005 and 2011 allowing for detailed
characterization of the gross dimensions. From these measurements it has been determined that Bennu possesses
a mean diameter of 492 ± 20 m with a total volume of 0.0623 ± 0.006 km3 and a surface area of 0.786 ± 0.04
km2 (see figure 1) [10]. The shape of the asteroid is roughly spherical with a top-like appearance and a maximum diameter of 565 ± 10 m. From the same observations the sidereal rotation period was determined to be
4.297 ± 0.002 hours and that the asteroid has a bulk density of 0.98 ± 0.15 g/cm3 [11].
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Obs. Start
Obs. End
30.7.1971
4.8.1971
Eng. Rng. [keV]
Eng. Res. [keV]
Det. Area [cm2 ]
FOV
Notes
1.5-5.5
—
25.0
30.0◦
3 collimated prop.
counters [13, 14]
Moon
14.9.2007
29.5.2009
1.0-10.0
0.180@5.9 keV
100.0
12.0◦ × 12.0◦
4 collimators each
with 2 × 2 CCDs
(Rad. Damage)
[15–17]
Hayabusa-XRS
25143 Itokawa
12.9.2005
24.11.2005
1.0-10.0
0.14@1.5 keV
25.0
3.5◦ × 3.5◦
1 collimator
with 2 × 2 CCDs
[18–20]
Chandrayaan-1 C1XS
Moon
11.2008
8.2009
0.8-20.0
0.2
24.0
12.0◦
24 SCDs in
3 collimators
[21, 22]
SMART-1 D-CIXS
Moon
3.2005
3.2005
1.0-20.0
0.2
12.0
12◦
24 SCDs in
3 collimators [23]
NEAR-XRS
443 Eros
4.5.2000
10.2.2001
1.5-5.5
0.83@5.9 keV
25.0
5◦
3 collimated prop.
counters [24]
Mercury Messenger
XRS
Mercury
2011
Present
1.0-10
0.88@5.9 keV
10.0
12.0◦
3 collimated prop.
counters [25, 26]
Bepicolombo
MIXS-C†
Mercury
8.2019
8.2020
0.5-10.0
0.128@5.9 keV
3.68
10.4◦
Single APS in a
single collimator
[27, 28]
Bepicolombo
MIXS-T†∗
Mercury
8.2019
8.2020
0.5-10.0
0.128@5.9 keV
3.68
1.1◦
APS w/Focusing optic
60 angular res.
[27, 28]
OSIRIS-REx
REXIS†‡
101955 Bennu
12.2019
3.2020
0.3-10.0
0.130@5.9keV
24.2
30.0◦
2 × 2 CCDs
Coded-aperture tel.
26.20 angular res.
Instrument
Target
Apollo 15 XRFS
Moon
Kaguya-XRS
•
•
Radiation Damage
Future Missions
‡
Coded-Aperture Telescope
∗
Focusing Telescope
†
Table 1: A list of previous remote sensing X-ray fluorescence experiments flown or scheduled to be flown for the
characterization of elemental abundances on airless bodies within the solar system.
Asteroids constitute the remaining building blocks of terrestrial planet formation and therefore provide an
important window into the conditions present during the formation of the Solar System. More specifically
primordial carbonaceous asteroids are a potential source of organic matter and other volatile elements, such
as sulfur, and may account for a large fraction of these elements on the Earth. Observations of Bennu are
characterized by a low albedo of approximately 0.035 ± 0.015 and a lack of absorption bands characterizing other
B-Type asteroids; the closest meteorite analog was determined to be that of a C1 and/or CM1 Chondrite [12].
3. THE REGOLITH X-RAY IMAGING SPECTROMETER
The REgolith X-Ray Imaging Spectrometer (REXIS) (shown in figure 2) was conceived as a student led project
whose primary goal is the education of science and engineering students who will participate in the development
of flight hardware in future space missions. Additionally REXIS also augments the observation capabilities of
the OSIRIS-REx mission at the high end of the electromagnetic spectrum which will enable characterization
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Coded Aperture Mask
Radiator
Collimator and
Mask Support
Tower
Detector Plane
And Housing
Electronics Box
Instrument Deck (Asteroid-Facing Surface)
Sun-Facing Surface
Solar X-Ray Monitor (SXM)
Silicon Drift Detector (SDD)
Figure 2: The REXIS instrument consists of the main
detector and collimator assembly and the electronics
box which are physically connected by 5 thermal isolation standoffs. The mating interface to the OSIRISREx spacecraft are located on the underside of the
electronics box. Shown to the upper left is the prototype coded aperture mask which makes detection of
elemental abundance enhancements at 50 m scales possible. The solar X-ray monitor (SXM) is mounted on
the sun-facing side of the spacecraft separately from the
primary telescope.
Physical Parameters
Total Mass (CBE)
4.4 kg
Total Power (CBE)
10.8 W
Focal Length
20 cm
Detector Plane
2 × 2 CCDs
Active Area
24.159 cm2
FOV
0.18 sr (30◦ circ.) (FWZI)
Angular Resolution
26.20 (5.6 m @ 730 m)
CCD Parameters
Type
MIT-LL CCID-41
Energy Resolution
130 eV @ 5.9 keV
Energy Range
0.3-10.0 keV (QE≥0.3)
Pixels
1024 × 1024
Pixel Dimensions
24 µm × 24 µm
Active Area
6.03880 cm2
Super-Pixel Dimensions 0.192 mm × 0.192 mm
Depletion Depth
45 µm
Optical Blocking
220 nm Direct Deposited Al
Operating Temperature ≤ −60◦ C
Mask Parameters
Thickness
100 µm
Composition
ASI-301 Stainless Steel
Pattern Diameter
98.304 mm (64 Pixels)
Open Hole Fraction
0.5
Pixel Pitch
1.536 mm
Support Grid Width
100 µm
Solar X-Ray Monitor (SXM)
Detector
Amptek XR-100 SDD
Active Area
5 mm × 5 mm
Energy Range
1-20 keV
Energy Resolution
125 eV @ 5.9 keV
Depletion Depth
500 µm
Optical Blocking
0.5 mil Be Window
Operating Temperature ≤0◦ C
Table 2: A summary of the critical instrument parameters for REXIS, the CCDs, the coded aperture mask and
the solar X-ray monitor (SXM) shown to the left in figure
2. The values given for the mass and power consumption
of the instrument are the current best estimates (CBE).
of the asteroid elemental abundances from a global scale down to 50 m, a capability unique to REXIS among
instruments of this type that have previously flown (c.f. table 1).
REXIS is designed to observe induced X-ray fluorescence lines emitted from the asteroid surface that arise as a
result of exposure to solar X-rays as well as the cosmic X-ray background (CXB). A number of previous missions
have employed this technique for the observation of airless bodies throughout the Solar System beginning with
the Apollo 15 X-ray fluorescence experiment for the observation of the moon [14]. More recent observations
of the Moon have been carried out by the D-CIXS mapping spectrometer on SMART-1 [23], the C1XS on
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Chandrayaan-1 [21, 22, 29], and the XRS aboard Kaguya [15, 16]. Additionally similar observations of two
separate asteroids 433 Eros and 25143 Itokawa have been carried out by the Hayabusa-XRS [19, 20] and NEARXRS [24] respectively, and the Mercury Messenger mission is currently in the process of mapping the elemental
abundances on the surface of Mercury [26]. To date the instruments that have flown have all employed a collimator
based instrument tuned to the specific orbital and target parameters imposed by the constraints of the individual
missions (see table 1). This allows for some reconstruction of the elemental abundances as a function of position
on the object under study, however the angular resolution is defined by the size of the collimator and is therefore
rather coarse compared to what is achievable utilizing focusing X-ray optics or a coded-aperture instrument.
Quantum Efficiency
A variety of detector planes have been used in conjunction with these missions. To date XRF experiREXIS Detector Quantum Efficiencies
ments flown by National Aeronautics and Space AdREXIS CCD
1.0
ministration (NASA): the Apollo 15 XRFS, NEARBare BI CCD
XRS, and the Mercury Messenger XRS have been limSXM
ited to the use of essentially the same instrument con0.8
sisting of 3 gas proportional counters (GPC) where one
GPC is outfitted with an Al filter, another is outfit0.6
ted with a Mg filter, and the third is simply equipped
with a Be optical blocking filter in order to deconvolve the Al-K, Mg-K, and Si-K emission lines in spite
0.4
of the poor energy resolution (880 eV @ 6.5 keV) of
these detectors. More recent efforts supported by the
Japanese Aerospace Exploration Agency (JAXA) have
0.2
focused on the use of 4 × 4 arrays of X-ray CCDs integrated into collimators and have the advantage of
0.0 -1
0
1
substantially improved energy resolution 140 eV @ 1
10
10
10
Energy [keV]
keV. The primary drawback of this method is the susceptibility of CCDs to radiation damage, in particular
from low energy proton induced charge transfer inef- Figure 3: The on-axis quantum efficiency curve of the
ficiencies (CTI), which prevented the success of one bare MIT Lincoln Laboratory CCID-41 and the preinstrument: the Kaguya-XRS. Until recently the ef- dicted performance of the REXIS flight CCDs after apforts of the European Space Agency (ESA) were fo- plication of the 220 nm thick Al optical blocking filter
cused on the use of swept charge devices (SCD) which and taking into account the effects of molecular surface
are non-imaging silicon based detectors similar to a contamination between 0.1 and 20 keV. The solar X-ray
CCD but with an altered electrode layout optimized monitor (SXM) quantum efficiency including the 0.5 mil
for rapid readout that have energy resolutions com- thick Be optical blocking window is also shown.
parable to CCDs. The ESA Bepicolombo mission to
Mercury, set to begin observations in August of 2019, will employ two XRF instruments: a telescope consisting
of a collimator and a Depleted P-Channel Field Effect Transistor (DEPFET) array detector plane (MIXS-C)
and an identical detector plane outfitted with a focusing optic (MIXS-T) [27, 28].
The primary REXIS instrument consists of a detector plane comprised of a 2 × 2 array of back illuminated
(BI) CCDs, MIT Lincoln Laboratory (LL) CCID-41, identical to those used in the X-Ray Imaging Spectrometer
(XIS) [30] on Suzaku [31], a joint astrophysics mission between NASA and JAXA, which have quantum efficiencies
greater than 0.75 between 0.4 and 6.0 keV (see figure 3) [32, 33]. The active area of the CCD is composed of
a 1024 × 1024 array of pixels with a pitch of 24 µm. There is a gap of 38 detector pixels (0.912 mm) between
the active areas of the individual CCDs to ensure the safe assembly of the detector plane which are aligned in
order to preserve the pixel pitch across the entire detector plane. A optical blocking filter consisting of a 220
nm aluminium layer deposited directly on the CCD is present to mitigate the undesirable background signal
that would otherwise be induced by optical/UV photons reflected from the surface of the asteroid and onto the
detector plane. The detector plane is housed in a thermally isolated, shielded box which itself is located inside
of the instrument main truss that supports a coded aperture mask located 20 cm above the detector plane. The
detector plane is passively cooled to less than −60◦ C by means of a radiator attached to one surface of the
main truss and is oriented toward deep space during science operations. This entire assembly sits atop and is
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affixed with low thermal conductivity standoffs to the REXIS electronics box which contains the instrument’s
support electronics. The mask itself is manufactured from a single piece of 100 µm thick ASI-301 stainless steel
with a random pattern of chemically etched open holes. The pitch of the individual mask elements is 1.536 mm
with an open hole fraction of 0.5 and a support grid running between the individual elements of 100 µm. The
support grid reduces the throughput to 0.437. In order to guard against radiation induced CTI and preserve
the spectral resolution of the detector plane the REXIS instrument will be outfitted with a radiation cover at
the aperture which will remain closed until observations commence. The detector plane is also equipped with a
number of 55 Fe calibration sources which are arranged to enable the monitoring of a subset of detector pixels
at the beginning and end of the CCD readout chain throughout observations of Bennu. The underside of the
radiation cover, mounted above the coded aperture mask, also contains an additional series 55 Fe sources which
will be used to calibrate and monitor all pixels in the detector plane prior to the opening of the cover for science
observations.
The solar X-ray spectrum is highly variable characterized by episodic flares and outbursts typically on 600
s timescale with longer durations on the order of days possible; on longer time scales changes in flux of up to 3
orders of magnitude is also possible. For this reason REXIS, as well as all other XRF experiments of this type,
must be equipped with an additional sensor for the measurement of the local solar X-ray spectrum in order to
properly interpret the data. To this end REXIS includes a separate solar X-ray monitor (SXM) which is attached
to the sun-facing side of the spacecraft and interfaces directly with the REXIS main detector electronics. The
SXM is constructed using a commercially available silicon drift detector (SDD) manufactured by Amptek read
out by custom designed electronics originally commissioned for use in the Neutron Star Interior Composition
ExploreR (NICER) [34, 35] that have been optimized for use with the REXIS instrument. The SSD has an
active area of 25 mm2 and is housed behind a 0.5 mil thick optical blocking filter composed of Be. In order to
maintain a nominal operating temperature less than 0◦ C under direct exposure from the sun a thermoelectric
cooler is located directly beneath the SDD. The fully assembled SXM will have a quantum efficiency greater
than 0.1 between 1.1 and 30.0 keV with an energy resolution of 125 eV at 5.9 keV (see figure 3) [35, 36].
The primary REXIS instrument in conjunction with the SXM will carry out observations of the asteroid
Bennu and measure the asteroid elemental composition (see §6) and conduct a search for regions of enhanced
abundance as described in §7.
4. OBSERVATION STRATEGY
After an initial cruise phase and survey (see §1) the OSIRIS-REx spacecraft will enter a roughly circular polar
terminator orbit with a radius of 1 km or 730±20 m above the surface of the asteroid with an orbital period of 27
hours. 11 hours of observation each day are currently allocated for nadir pointing observations during which time
the REXIS field of view will be centered on the asteroid. The position of the REXIS-SXM has been optimized
for this observation period so that the sun remains in the SXM field of view to enable continuous monitoring of
the solar X-ray spectrum throughout science operations. Concordantly a radiator affixed to the main collimator
and mask support structure for passive cooling of the CCDs to less than -60◦ C has been mounted so that it is
oriented into deep space during these observations as well.
The CCDs in the detector plane are read out with an integration time of 4 seconds per frame, and a frame
transfer time of 10 ms. The individual frames are processed on board where the individual events from each
frame are extracted, graded and placed in an event list with a time tag before being staged on board the OSIRISREx spacecraft for downlink. The 4 second timing resolution of the detector plane combined with the asteroid
rotation period and nominal orbit of OSIRIS-REx during science operations set the lower bounds for the angular
resolution of the REXIS instrument at 3.60 , equivalent to a 28 cm spatial resolution on the surface of the asteroid
from an altitude of 730 m. The maximum spatial resolution of the detector plane is fixed by the CCD pixel size
of 24 µm × 24 µm, however, since this level of fidelity is not required pixels are re-binned in the on board data
processing to super pixels consisting of a 8 × 8 array of native pixels with a pitch of 0.192 µm. This is done
primarily to reduce the data volume of the instrument since the full spatial resolution of the detector plane is
not required to achieve REXIS’s science objectives
Science observations will commence just after the next solar minimum during which time a very low incident
solar X-ray flux is expected at the level defined by the Geostationary Operational Environmental Satellite (GOES)
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mission A-state, i.e 10−8 W/m2 in the 1-8 Å band solar X-ray monitor [37]. In order to mitigate this the REXIS
field of view has been maximized to enable the collection of as many X-ray events as possible while maintaining
minimal exposure to the cosmic X-ray background which is largely obscured by the asteroid in the REXIS field
of view. This comes at the expense of reduced spatial resolution on the surface of the asteroid for a collimator
based analysis. In an effort to measure elemental abundance with high spatial resolution (26.20 ) a coded aperture
mask with a open hole fraction of 0.5 and pixel pitch of 1.536 mm will be affixed at a distance (focal length) of
20 cm from the surface of the detector plane but reduces the total aperture throughput (see figure 2) by a factor
of two. Coded aperture instruments have been used in a variety of astrophysics missions, in particular wide-field
hard X-ray monitors sensitive at energies between approximately 15 and 200 keV such as the Swift Burst Alert
Telescope (BAT) [38] and the Imager on-board INTEGRAL (IBIS) [39] for the localization, long-term monitoring
and characterization of astrophysical point sources. This method was pioneered as a way to image hard X-rays
with energies above approximately 10 keV where the use of focusing optics was not possible until recently. In the
case of REXIS, which is sensitive between 0.4 and 10 keV, the use of a coded-aperture mask was born primarily
out of budget, mass constraints and FOV.
REXIS will be the first application of the coded aperture imaging technique to extended sources. Since the
surface being imaged provides the very background against which enhanced emission features are sought, the
technique is less sensitive than for point sources (for which it has been traditionally used). However, combined
with ”collimator mode” imaging (see §8), the coded aperture technique will reveal localized features above some
minimum enhancement value. A coded aperture imager operates as a pinhole camera, where additional pinholes
have been added in order to increase the total throughput of the instrument. The reconstruction of images from
coded aperture instruments proceeds directly from analysis of the X-ray event list by selecting events with the
energies within the band of interest then generating detector plane images using the spatial information recorded
in the individual CCDs for each exposure. The reconstruction of the local field for each individual exposure
is achieved by convolving the detector plane image with the mask image as described in [40] and in §7; this is
analogous to a search for the presence of significant projections of the mask pattern on the detector plane from
all possible angles in X-rays. A final asteroid image is achieved by co-addition of the individual reconstructed
sky images reprojected onto the surface asteroid surface (see §7 for details). Finally the angular resolution of a
coded aperture instrument is defined by
!
p
p2m + p2d
−1
(1)
δφ = tan
f
where f is the separation between the detector plane and the coded-aperture mask (i.e. focal length), pm and pd
is the pitches of the mask and detector pixels respectively. The angular resolution of the REXIS instrument is
26.20 , about a factor of 7.3 below the maximum achievable based on the CCD integration time and the motion
of the spacecraft and asteroid. This is equivalent to a spatial resolution on the asteroid surface of 5.6 m at an
altitude of 730 m.
5. ASTEROID X-RAY EMISSION MODELING
To evaluate the performance of REXIS spectral modeling was carried out primarily using the composition of the
nearest meteorite match to the observed spectrum of 101955 Bennu (see §2): that of a C1 chondrite. The single
interaction model presented in [41] was adopted and cross checked with an independent GEANT4.3 simulation.
The current calculation assumes a smooth spherical asteroid with a uniform density of 1 g/cm3 (see §2).
The input solar spectrum for baseline simulation was chosen using data from the GOES X-ray monitors [37]
in order to estimate the expected temperature and emission measures around the depth of solar minimum, during
which REXIS will carry out observations. Using data extracted for the GOES-10 and GOES-12 X-ray monitors
averaged over 5 minute intervals and the method introduced in [42] emission measures and temperatures from
the time around the previous solar minimum were derived. Taking a simplistic, conservative approach derived
emission measures and temperatures from the GOES-10 and GOES-12 satellites were compared and data points
found to differ by more than 5% were excluded from consideration. Cumulative probability distributions were
calculated for both (see figure 4) and a worst-case scenario temperature 4 MK and an emission measure of 1044
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Temperature Cumulative Probability
1.0
X-Ray Monitor
GOES-10
GOES-12
0.6
0.4
0.6
0.4
0.2
0.2
0.0
4
GOES-10
GOES-12
0.8
Cumulative Probability
Cumulative Probability
0.8
Emission Measure Cumulative Probability
1.0
X-Ray Monitor
6
8
10
12
Temperature [MK]
14
16
18
0.0 39
10
40
10
41
10
42
10
43
10
44
10
45
10
46
10
47
10
48
10
49
10
50
10
Emission Measure [cm¡2 ]
Figure 4: The cumulative probability distribution of the measured temperature and emission measures of the
sun using data from the GOES-10, and 12 X-ray monitors during the time around the previous solar minimum
between January 1, 2005 and January 1, 2007: the closest analog for the REXIS observing period. A cut on the
data has been implemented requiring agreement between the monitor data of 5% in order to exclude instrumental
and environmental effects.
cm−3 were adopted and used to generate single temperature models using CHIANTI 7.1 [43, 44]. Additionally
these “worst-case” values were compared to results from Reuven Ramatti High Energy Spectroscopic Imager
(RHESSI) which place constraints on the solar X-ray spectrum at solar minimum during periods where the
GOES 1-8 Å monitors register solar activity below 10−8 W/m2 , i.e. below the A-state, and found to in be good
agreement with this choice of parameters [45]. In addition coronal plasmas at higher temperature states were
also considered for the imaging simulation (see §7).
After calculation of the solar spectrum incident on the asteroid, the induced X-ray fluorescence (XRF) spectrum is derived
for a simple spherical asteroid by numerical integration. To accomplish this a map of the asteroid is divided into equal area bins
using an AITOFF-Hammer projection, identical to that used in
the imaging routines described in §7. The incidence angle and reemission angle to the position of the OSIRIS-REx spacecraft are
derived for each bin then, using the prescription in [41], the coherent and incoherent scattering intensities as well as the intensity of
Table 3: The estimated minimum detection the individual fluorescence lines are calculated. The incoming flux
times for key elements for an asteroid with is then integrated over all bins visible to the REXIS instrument
the composition of a C1-Chondrite at a sig- weighted by the response function of the coded-aperture mask
shown in figure 7a giving the total solar induced XRF spectrum.
nificance of 6σ.
Line
Min. Det. Time
Fe-L
136 sec.
Mg-K 63 sec.
Al-K
7.67 hrs.
Si-K
183 sec.
S-Kα 5.73 days
Internal bkg. lines not included
In addition the XRF spectrum induced by the cosmic X-ray
background (CXB) is calculated in a similar fashion assuming a
completely uniform CXB. Under this assumption each bin on the asteroid surface receives the same incident flux
from a 2π radian region on the sky. Again following [41] but integrating over all possible incidence angles each
component of the XRF spectrum is calculated for a fixed spacecraft position giving the total CXB induced XRF
spectrum. Combination of these two components give the total predicted count rate for each line of interest
during low solar activity incident on the surface of the CCDs. For this time period the CXB induced XRF
emission becomes important for elements at energies above 3 to 4 keV. For this calculation particle induced
X-ray emission (PIXE) is not yet included but will be incorporated into future models.
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Abundance Ratios for Various Meteorite Groups
1.6
Abundance Ratios for Different C-Chondrite Subtypes
Meteorite Class
Meteorite Group
[Mg/Si]
1.5
1.0
IAB-Winonaite
L-Chondrite
LL-Chondrite
Lodranite
Lunar
Mesosiderite
Pallasite
Ureilite
Other
C-Chondrite
REXIS Requirement
Predicted Perf.
Other
C
C1
CI1
CM2
C4
C6
CK4
CK5
CV3
1.4
1.2
1.0
[Mg/Si]
REXIS Requirement
Predicted Perf.
Acapulcoite
Angrite
Aubrite
Brachinite
Diogenite
E-Chondrite
Eucrite
H-Chondrite
0.8
0.6
0.5
0.4
0.2
CV
0.0
0
CM
CI
0.0
1
2
3
4
5
−0.2
0.0
0.2
[Fe/Si]
(a)
0.4
[S/Si]
0.6
0.8
1.0
(b)
Figure 5: Ground observations have shown Bennu to most closely resemble a C1 or CM chondritic meteorite. Two
of the primary objectives of the REXIS mission is to (a) determine whether or not Bennu is indeed a Chondrite,
and to (b) determine which type of Chondrite based on observations of Sulfur. The expected performance
and requirement values are show in the above plots for REXIS as a dashed and solid line respectively. The
measurements of the elemental abundance ratios for the individual meteorites were carried out in the lab and
the results are given in [50].
Next the attenuation due to the presence of the 220 nm thick Al optical blocking filter and due to the
presence of organic contaminants on the surface of the CCD is applied along with the quantum efficiency of
the detectors as a function of energy (see figure 3) to produce a predicted count rate as a function of energy
for the entire REXIS instrument [46]. The collection of contaminants on the surface of the CCD is mitigated
by the implementation of pre-flight handling procedures, however the migration and accumulation of organic
contaminants on optical blocking filters and detectors consisting primarily of carbon has been observed in both
the Chandra X-ray observatory Advanced CCD Imaging Spectrometer (CXO-ACIS) [47], the Suzaku XIS (c.f.
[48]) and many others. Such contaminants are suspected to originate from the spacecraft itself [49] with an
accumulation rate has been observed to have a strong temperature dependence. Therefore the final contamination
properties will vary from mission-to-mission depending upon the components, geometry and temperature of the
spacecraft; difficult to assess prior to launch. Based on the observed properties of the CXO-ACIS and Suzaku XIS
contamination layers a representative contamination layer consisting of C and O with a atomic ratio C/O=6 and
total thickness of 54 µg/cm2 has been assumed for our simulations. The minimum detection times for elements
key to the identification of Bennu outlined in §6 are given in table3.
6. METEORITE CLASSIFICATION
The primary objective of the REXIS instrument is the identification 101955 Bennu elemental surface composition
through the observation of the Fe-L, Al-K, Mg-K, and Si-K complexes, as well as the S-Kα and S-Kβ fluorescence
lines. Using data from [50] it was determined that the primary means of identification will be carried out through
measurement of the mass abundance ratios [Mg/Si] and [Fe/Si] (see figure 5a) which will enable REXIS to
test previous observations of Bennu (see §2). Abundance ratios are typically used in place of simple element
abundances as these are relatively insensitive to changes in solar states and unexpected changes in instrument
performance. In order to definitively classify Bennu as a carbonaceous chondrite REXIS is required to measure
these abundance ratios to within 20% of their true value at a confidence level of 10σ; the current best estimate
for the expected performance of the instrument now stands as 18.8% [51].
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−4
−2
0
2
4
6
8
Signal-to-Noise Ratio
10
Figure 6: The complete reconstructed, simulated image of Bennu with a single 50 m scale source with a factor
of 5 abundance enhancement observed with a total integration time of 20 days. The peak signal-to-noise ratio is
7.8; a simple scaling indicates that a detection at the 6σ level is expected for factors of 3.75 enhancement in Fe
abundance.
REXIS is not only designed toward determining if Bennu is a carbonaceous chondrite, but will also be able to
distinguish the subgroup to which Bennu belongs. To accomplish this the [Mg/Si] and [S/Si] abundance ratios
are required to be measured (see figure 5b) to within 20% of their true value with an expected performance of
12%.
7. CODED APERTURE IMAGING SIMULATION AND RECONSTRUCTION
An imaging simulation has been carried out for a fixed orbital radius of 1 km around a spherical asteroid with
a radius of 280 m in the Fe-L and Mg-K bands in order to assess the sensitivity of the REXIS instrument to
enhancements of elemental abundance on 50 m scales on the asteroid surface. Initially a flux map of the asteroid
is initially generated on a AITOFF-Hammer equal area projection and regions of enhanced flux are added to the
map simulating regions with enhanced elemental abundance.
The imaging simulation proceeds in two step: first the collection of data in 600 second time steps on the
detector plane is simulated and a detector plane image generated at each step. This is coarser than the 4 second
integration time of the REXIS detector plane (see §3) in order to carry out the simulations within a short time
frame: approximately 48 hours for 20 days of simulated observation per CPU core. Next the individual detector
plane images are used to reconstruct individual observation images which are then co-added over the surface of
the asteroid in a AITOFF-Hammer projection. A search over this final map is then conducted for statistically
significant flux enhancement.
The data collection simulation is carried out by initially calculating the sub-solar point and the position of
the OSIRIS-REx spacecraft at each step as a function of time fully taking into account the orbit of OSIRIS-REx
as well as the rotation of 101955 Bennu. At each simulation step the solar flux incident at each point on the
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asteroid is first calculated which is then used to calculate the intensity of the X-ray emission from the surface of
the asteroid to the REXIS instrument. The intensity map is then re-projected into the instrument’s local field
of view on a tangential projection and convolved with the coded aperture mask using a fast Fourier transform
(FFT) producing an incident intensity map on the REXIS detector plane. This map is then filtered according
to the active area of the CCD and the expectation value of the number of counts in each CCD super-pixel
calculated. A simulated observation is then carried out by sampling each pixel on a Poisson distribution using
the expectation value producing a single detector plane image. The detector plane image together with the
spacecraft position and sub-solar position are stored to disk at each step.
The image reconstruction process is carried out first by reconstructing the sky image from the stored detector
plane image through an FFT convolution with the coded-aperture mask using a balanced correlation where open
mask pixels are assigned a value of 1 and closed mask pixels are assigned a value of ρ/(ρ − 1) where ρ is the open
hole fraction [40]; mask pixels outside of the circular mask pattern area are removed from the reconstruction by
setting their values to 0. Similarly the sum of all events over all pixels in the detector plane active area is set
to 0 by subtraction the mean number of events taken over all active elements. In carrying out this procedure
an automatic background subtraction of the reconstructed sky image is carried out giving an excess map in sky
coordinates. Using the OSIRIS-REx spacecraft position the sky image is reprojected to asteroid coordinates
on an AITOFF-Hammer equal area projection. The equal area projection is used in order to carry out the
subsequent analysis on equal sized surface area elements on the asteroid surface as well as to co-add images in a
coordinate space that is independent of the spacecraft view factors to the asteroid surface.
In addition a background estimation procedure is also carried out over the asteroid surface by random reassignment of the positions for individual sky images before re-projection within a fixed time interval around individual images. This time-shuffling procedure has been introduced in order to remove the large-scale anisotropies
associated viewing the bright limb of the asteroid and the attendant effect within the co-added sky images in
asteroid coordinates. The same goal can be achieved in the detector image space by co-adding detector plane
images over a rolling time window to produce a background detector map which can then be subtracted from
the individual detector plane before reconstruction of the sky image (see figure 6). Although the variability of
the solar flux has not yet been added to this imaging simulation the purpose of a subtracting images at nearby
times is to test the procedure that will be required when dealing with long-term variability of the solar state over
the REXIS observation period. In the final analysis procedure short-term variability will be handled using input
from the SXM to sort individual images into separate solar states for co-addition into separate asteroid maps
for each state in order to avoid the appearance of overabundant regions due only to enhanced fluxes caused by
solar flares in individual images.
Initially the imaging simulation was carried out assuming a 4 MK coronal temperature with an emission
measure of 1044 cm−2 , however it was subsequently determined that the flux enhancements required for the
detection of a source on the surface of a C1 Chondrite was unphysical. It was determined shortly thereafter
that enhancements Fe and Mg elemental abundance under exposure from a 6 MK sun would yield detectable
sources on 50 m scales. Since approximately 95% of observations are likely to be carried under exposure from a
T ≥ 6 MK sun (see figure 4 and §5) assuming the same conditions as the previous solar minimum: equivalent
to 19 days of observation, however intervals of time during which the short wavelength GOES X-ray monitors
(0.5-4 Å) were unable to measure significant fluxes from the sun at low solar states were excluded. Assuming
there periods represent lower coronal temperatures the expected REXIS observing time during 6 MK solar state
requires a downward correction of up to 50%. A more careful evaluation of this is currently underway [52].
8. COLLIMATOR MODE SIMULATION AND RECONSTRUCTION
The simulation and reconstruction of images in collimator mode is carried out in a similar manner to that of
the imaging simulation and reconstruction described in §7. The key difference is that the response function of
the entire REXIS instrument (see figure 7a) is used rather than individual mask pixels for the reconstruction of
individual asteroid images. This leads to degraded angular resolution but somewhat higher sensitivity to larger
scale elemental abundance enhancements on the asteroid surface. Additionally the terminator orbit configuration
for the REXIS observations also proves to be advantageous, particularly for lower energy elements where the
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0.0
0.1
0.2
0.3
0.4
0.5
Coded-Aperture Mask Throughput
(a) The REXIS Instantaneous Response Function
−4
−2
4
0
2
6
8
Signal-to-Noise Ratio
10
(b) Reconstructed Collimator Mode Image
Figure 7: (a) The response function of the REXIS instrument is defined by the coded aperture mask throughput
shown for REXIS during nadir pointing at (lat, lon) = (−90.0, 0.0). For reconstruction of elemental abundance
maps at low energies the response function can be further reduced by weighting the exposed areas by the
intensity of the solar exposure. (b) The reconstructed image of elemental abundance enhancements for a single
Fe enhancement located on the surface of 101955 Bennu identical to that detected in figure 6. The peak signal
to noise of the detected source 10.2σ.
relative contribution of the CXB induced X-ray fluorescence is low and the emission can be approximately
considered to originate from one half of the REXIS field of view.
In order to simulate the collimator mode reconstruction the same simulated data set consisting of detector
plane images, the OSIRIS-REx spacecraft position, and sub-solar position as a function of time is generated
as described in §7. The position of each detector plane image is then used to project the collimator response
function onto the surface of the asteroid weighted by the total number of events registered in the detector plane
over the energy band of interest. For low energy elements whose fluorescence lines are primarily induced by solar
X-ray the emission can be considered to have originated from the illuminated half of the asteroid. Weighting of
the response function by the incident solar exposure, effectively cutting the response function in half, improves
the collimator mode spatial resolution by approximately a factor of two.
A similar method for the estimation of a background map is also carried out by time shuffling collimator and
co-adding images on the asteroid surface as was described in §7. As before a time window of 12000 seconds,
equivalent to 20 exposures, was adopted. After generation the raw asteroid map is subtracted from the background asteroid map giving excess as a function of position with a spatial resolution on the asteroid surface of
250 m.
Studies are underway to further characterize the performance of this reconstruction technique for the REXIS
instrument as a function of source strength and extension [52], however low spatial resolution detections of source
on the asteroid surface have been simulated resulting in high signal-to-noise detections for Fe sources on scales
of 50 m and larger with a factor of 5 element abundance enhancement (c.f. figure 7b).
9. OPTIMIZATION OF THE REXIS IMAGING SYSTEM
The REXIS instrument (see figure 2) coded aperture mask and imaging system has been optimized using the
imaging simulation described in §7. The primary parameters for optimization were the radius of the mask
pattern, the mask open hole fraction, and the pixel pitch of the mask as well as that of the detector plane; see
table 2 for a list of the current parameters.
The optimization procedure was as follows: the input parameters for a candidate mask are set, i.e. pattern
radius, pixel pitch, support grid width, and open hole fraction, each mask pattern was optimized through
generation of 1000 candidate masks. Next a extreme high flux point source simulation using the REXIS detector
plane with a pixel pitch of 0.368 mm and iterating the source over the entire REXIS field of view in order
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to measure the maximum achievable signal-to-noise ratio. From the sample of 1000 candidate masks the mask
showing the best performance over the entire field of view is chosen for use in the subsequent simulations discussed
below.
The first parameter considered for optimization was the mask and detector plane pixel pitch. A series of
simulations for the full observation period of REXIS was carried out for a test detector with pixel pitch of 0.368
mm and a mask pixel pitch of 1.152 mm with a region of enhancement identical to that described in §7. This
leads to an improvement in the angular resolution from 26.20 to 20.70 at the cost of a 7% decrease in the total
count rate resulting from a decrease in the mask throughput. This is due to the chemical etching process used
for the production of the coded aperture mask which requires that the support grids running between individual
elements is are at least equal to the thickness of the material; in this case 100 µm. The on-axis throughput of
a coded aperture mask with support grids can be calculated by T 0 = T (1 − g/mp )2 , where T is the throughput
expected for a perfect mask without support grids, i.e. the open hole fraction, mp is the mask pixel pitch and g is
the width of the support grid. For the test carried out here a 4.8% decrease in the on-axis throughput, from 0.437
to 0.417, occurs in the transition from the 1.536 mm mask to the 1.152 mm mask. Although this will not have
a significant negative impact on our observations the 1.536 mm mask was retained since a significant positive
impact was also not observed. From a fabrication and integration perspective the tightening of the alignment
requirements for the use of the finer pitch mask was not warranted given the lack of any clear advantage.
It was recognized early on that the performance of the spectral observations could be improved by increasing
the throughput of the coded aperture mask at the expense of imaging sensitivity. In order to ascertain the effects
on imaging the same simulation was run using coded aperture masks with ideal open fractions of 0.25, 0.35,
0.4, 0.6, 0.65, and 0.75. A series of 10 simulations were run for each mask pattern in order to measure not only
the peak signal-to-noise ratio of each source but also to check the fluctuation level of the result. No significant
detections were found in any of these test runs. Additional simulation were initiated for masks with open fraction
of 0.45 and 0.55 in a search for potential optimization of the image reconstruction unfortunately the result here
was also negative, confirming the open hole fraction of 0.5 to be optimal for the observation of Bennu.
10. DISCUSSION
The REXIS instrument has been optimized for observation of the near-Earth asteroid 101955 Bennu as part
of the OSIRIS-REx mission and seeks to identify a meteorite classification of Bennu through measurement of
elemental abundances by observation of solar X-ray and CXB induced fluorescence lines emitted from the surface
of the asteroid. REXIS will also search for localized areas of enhanced elemental abundance with scale sizes of
approximately 50 m in order to aid in the selection of a sample collection site using a coded aperture mask
capable of resolving features as small as 5.6 m on the surface of the asteroid. REXIS will be the first instrument
to employ this technique for use on a planetary mission and owing to its large field of view will also maintain
similar capabilities to instruments which have previously been used in this capacity.
ACKNOWLEDGMENTS
This work was supported under NASA Grants NNX12AG65G and NNG12FD70C. We would like to thank Martin
Elvis for making the initial contact between the Harvard College Observatory and Massachusetts Institute of
Technology groups which have participated in the development of REXIS, Lucy Lim for providing invaluable
advice on the utility of solar X-ray monitors, George Sondecker for his work during the proposal and early phase
of this project, as well as Kevin Ryu and Vsyhi Suntharalingam for the preparation of the REXIS CCDs.
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